Staphylococcal protein A (SpA), is a 42 kDa multi-domain protein from the bacterium Staphylococcus aureus. SpA is bound to the bacterial cell wall via its carboxy-terminal cell wall binding region, referred to as the X domain. At the amino-terminal region, it includes five immunoglobulin-binding domains, referred to as E, D, A, B, and C (Sjodhal, Eur J Biochem. September;78(2):471-90 (1977); Uhlen et al., J Biol Chem. February 10;259(3):1695-702 (1984)). Each of these domains contains approximately 58 amino acid residues, and they share 65-90% amino acid sequence identity. The Z domain of SpA is an engineered analogue of the B domain of SpA and includes an alanine instead of a glycine residue at position 29.
SpA based reagents have found a widespread use in the field of biotechnology, e.g., in affinity chromatography for capture and purification of antibodies as well as in antibody detection methods. At present, SpA-based affinity media probably are the most widely used affinity media for isolation of monoclonal antibodies and their fragments from different samples including cell culture. Accordingly, various matrices comprising protein A-ligands are commercially available including, for example, ProSep®-vA High Capacity, ProSep® vA Ultra and ProSep® UltraPlus (Millipore) and Protein A Sepharose™, MabSelect™, MabSelect Xtra™ and MabSelect SuRe® (GE Healthcare).